1. Field of the Invention
This invention relates generally to interface circuits for use in telephone systems and more particularly to a trunk interface circuit between a PABX or cental office (C.O.) line circuit and a telephone set.
2. Description of the Prior Art
Each telephone set is connected to a central office that contains switching equipment, signaling equipment, and batteries that supply direct current to operate the telephone set and is connected to the central office through a trunk comprising two wires, termed a local loop, or alternatively to a PABX interconnected with the central office. One of these wires in the wire pair is designated T (for "tip") and the other is designated R (for "ring"). The tip-ring interface is located between the telephone set and the terminus of the local loop. This interface contains signaling circuitry--for going off-hook, for dialing, and for receiving incoming ringing and forward and reverse loop current; transmission circuitry--for the transmission and reception of audio signals, i.e., the circuits that carry our voices; and isolation circuitry--for electrically isolating the telephone set from the central office. The interface must include detecting circuitry for detecting ringing and the presence of current in the loop in either the forward or reverse direction.
Previous interface circuits used separate detectors for ringing signal detection, forward loop current detection, and reverse loop current detection. The interface curcuits reflect per line expenses and, thus, any increase in the costs of an interface is multiplied by the number of lines utlized by a customer. The cost of these extra detectors and associated circuitry thus becomes a significant cost to the customer. Additionally, a system utilizing two detectors to sense loop directions presents two output signals which must be processed thereby further increasing the cost per line of prior art systems.